While the preferred fuel and engine for an automobile traditionally have been gasoline and an internal combustion engine, respectively, newer cars are providing the ability to run their engines on electricity with power achieved from a battery pack assembly placed within the car. Electric automobiles are less prone to noise and are not as polluting to air as are their gasoline-based counterpart cars.
An important distinguishing aspect of electric cars is their battery pack apparatus. Critical design parameters include the cost of the battery per kilo watt hour (KWH), the number of miles the car can be driven without a recharge, the safety of the driver from battery thermal runaway or explosion, and the cost and efficiency of the cooling systems for the battery pack. The cooling system efficiency also affects the cost per KWH, since the higher the operating temperature rises due to inefficient cooling, the more design cost and effort is incurred to make car components resistant and reliable to heat and rising temperatures as well as battery cell life.
Traditionally, a battery module for an electric car had battery cells arranged in rows, and had a straight (linear) coolant pipe placed against each row of battery cells. The battery cells generally are cylindrical, so that they have circular cross-sections. Consequently, the area of contact between the coolant pipe and each battery cell was very small, approximating that of a point (i.e., the point at which a circle touches a line tangential to the circle), thereby restricting the conduction of heat from the battery cell to the coolant. That design subsequently was improved slightly, by making the coolant pipes locally curvilinear at each cell, to better match the contours of the battery cells. This increased the contact area and hence increased heat conduction from the battery cell to the coolant pipe. However, the angle of contact between the pipe and each cell still remained only on the order of about 15 degrees, so that heat conduction from the battery cell to the coolant pipe was still restrictive. Furthermore, the coolant at the end of the pipe is at a higher temperature at the entrance of the pipe, resulting in a temperature difference between the first and last battery cell.
Due to poor heat conduction, battery cells have been vulnerable to increase in temperature resulting in fire or explosion. Additionally, accidental collision cannot be ruled out even in the most conservative and advanced electric cars, putting the electric car and occupants at risk from fire or explosion of the battery pack due to collision. The energy density of battery cells is expected to double every few years, with 10 times current energy density being developed in university laboratories. The higher the energy density is, the more critical is the cooling system to control the temperature of the battery cells for both vehicle safety and longevity of the battery cells.